William and Kate traveled by car from Kensington Palace to St. Mary's Hospital, where they entered through a side door and dodged the world's media. About 90 minutes later, palace officials confirmed her arrival in a statement.
A spokesman for the couple said The Duchess of Cambridge's labor was progressing normally.
Kate and William have chosen to be surprised and do not know the sex of the baby, according to royal sources.
Kate is expected to give birth in the private Lindo Wing of the hospital. That is the same location where Princess Diana gave birth to Prince William in 1982 and his younger brother, Prince Harry.
The 31-year-old will be looked after by a top-notch medical team led by royal gynecologist Dr. Marcus Setchell and assisted by Alan Farthin. Palace officials are not expected to provide many details about how her labor is progressing.
Royal officials were forced to announce the pregnancy in December after the Duchess' admission into the hospital for hyperemesis gravidarum, a serious condition that causes severe vomiting and dehydration.
Whether a boy or a girl, the child of Kate and Prince William will be third in line to the British throne and is anticipated eventually to become king or queen.
The child will be the first for William and Kate, who married in 2011 after a long courtship.
Royal watchers must wait to be told of the baby's arrival from the palace, which is planning to reveal the news through a mixture of tradition and social media.
Palace officials have said that the first hint will come when a royal aide emerges from the hospital with a signed bulletin carrying the Buckingham Palace letterhead. The bulletin will be given to an official who will be driven to Buckingham Palace, where it will be posted on an easel in public view in front of the building.
At the same time the bulletin is posted, there will be an official announcement on Twitter and the media will be formally notified. The document will give the baby's gender, weight and time of birth.
The Associated Press and ABC News contributed to this report.